BERKEY, GEOLOGICAL RECO:^NOISSANCE OF PORTO RICO 49 



vation. The presence of gi-eat quantities of roughly assorted gravels 

 clogging some of the valleys of the southerly side of the island tends to 

 support the same general conclusion. The bearing of these features on 

 the geological history of Porto Rico will be taken up at another point. 



Cuestas 



Both on the north side, for nearly the whole length of the island, and 

 on the south side, over the westerly half, there is a bordering belt of 

 limestone and associated beds that bave been developed on an eroded sur- 



FiG. 17. — Structure beneattt the marginal terraces 



Strongly bedded ash together with associated shales cut by small dikes forming a part 

 of the terrace uear Guayama. These rocks belong to the older series and dip into or 

 toward the mountains rather than toward the sea. 



face which beveled across the more complex structures of the older series 

 of formations that fonnerly made up the mass of the island. These 

 limestone beds are several hundred feet in thickness and dip gently 

 toward the sea. On the inner margin of their present extent toward the 

 interior, especially along the north side of the island, they are abruptly 

 terminated in a very irregular line of modified cliff forms facing toward 

 the prevailingly smoother and lower ground for some distance toward 

 the interior. For tlie most part, this limestone margin is exceedingly 

 rugo^ed and broken. The width of the lielt with this rugged character 



